A computer uses a volatile memory such as a dynamic random access memory (DRAM) and a static random access memory (SRAM), or the like for its main storage. The volatile memory can retain data only while electric power source is supplied, and loses the stored data when the power source supply is turned off. Meanwhile, recently, as a nonvolatile memory, which is freely rewritable and does not lose data even if the power source supply is turned off, a ferroelectric random access memory (hereinafter referred to as a “FeRAM”) utilizing a ferroelectric film has gathered much attention. The FeRAM has advantages in that it can be highly integrated and consumes low electric power in addition to it is a nonvolatile memory.
Conventional FeRAMs are provided with a ferroelectric capacitor (hereinafter also referred to as a “FeCap”) 205 configured by sandwiching a ferroelectric film 203 between two electrodes 204 and 202, as shown in FIG. 15. The FeCap 205 is formed, for example, above a substrate 201 via an insulating film and the like (not shown). The FeCap 205 constitutes a functional main part of the FeRAM. The FeCap is also used for a sensor, sometimes. The electrodes 202 and 204 sandwiching the ferroelectric film 203 are called a lower electrode and an upper electrode, respectively.
As for materials of the ferroelectric film and the electrode, many research studies have been carried out and thereby various materials are proposed. For the ferroelectric film 203, Pb(Zr,Ti)O3 (hereinafter also referred to as a “PZT”), (Bi,La)4Ti3O12, SrBi2Ta2O9, or the like are mainly used.
Further, as an upper electrode, that composed of a Pt film is mainly used, however, that composed of a composite film of a SrRuO3 film and a Pt film, that composed of a conductive oxide film such as a (La,Sr)CoO3 film or an IrO2 film, and so forth have also been researched.
For improving storage density of such a FeRAM, the FeCap is required to have stable characteristics. As an important characteristic of the FeCap, there is a polarization characteristic. The polarization characteristic degrades due, for example, to fatigue of the ferroelectric film. This type of fatigue is observed when the ferroelectric film is repeatedly switched by an electric field many times. Although the ferroelectric film is required to have an endurance to 1012 cycles of the switching electric field nowadays, the endurance upto 1015 cycles of the switching electric field is considered to be required in the future. Further, the FeCap must also have a low leakage current.
However, when the upper electrode composed of a composite film consisting of SrRuO3 film and Pt film is utilized, the SrRuO3 film itself does not function as an electrode, so that the stacked structure with the Pt film is employed. Therefore, as compared to the upper electrode composed only of the Pt film, the manufacturing processes increase in number and the manufacturing time may increase.
Further, there is a report stating that the fatigue becomes more difficult to be caused when the upper electrode composed of the conductive oxide film such as the (La,Sr)CoO3 film or the IrO2 film is utilized. However, the mechanism has not yet been figured out, and whether or not the fatigue actually becomes difficult to be caused is still uncertain.
Further, the combination of the ferroelectric film composed of the PZT and an upper electrode composed of the IrO2 has also been examined (Patent Document 4, Patent Document 5 and Non-Patent Document 1 to 3), however, optimized characteristics have not yet been obtained, so that further improvements are expected. Still further, in the methods described in Patent Document 4 and Patent Document 5, after the ferroelectric film is crystallized, the surface layer thereof is needed to be removed, so that the manufacturing process is complicated.
(Patent Document 1) Japanese Patent Application Laid-Open No. 2001-127262
(Patent Document 2) Japanese Patent Application Laid-Open No. 2000-260954
(Patent Document 3) United States Patent No.
(Patent Document 4) Japanese Patent Application Laid-Open No. Hei 10-341010
(Patent Document 5) United States Patent No.
(Non-Patent Document 1) T. Nakamura, et al., Jpn. J. Appl. Phys. 33, 5207 (1994)
(Non-Patent Document 2) K. Kushida-Abdelghafar, et al, J. Appl. Phys. 85, 1069 (1999)
(Non-Patent Document 3) T. Sakoda, et al., Jpn. J. Appl. Phys. 40, 2911 (2001)